One approach to measuring the distance between two points, without the disadvantages of using mechanical measuring instruments such as tape measures, is to use an acoustic ranging apparatus. In one example, this ranging apparatus involves directing a burst of acoustic energy, such as ultrasonic energy, from a source to an acoustic energy receiver and measuring the time it takes for the energy to traverse the distance between the source and the receiver. In another example of an acoustic ranging apparatus, the distance between a source and an acoustically reflective object may be measured by directing a burst of acoustic energy from the source toward the object. The total time it takes for the burst to traverse the distance between the source and the object and for a reflection of that burst from the object (an echo) to travel to a receiver at a predetermined point fixed with respect to the source is measured. The receiving point may be at the source itself in a situation where the transducer which produces the burst is also the receiver which detects the echo. The receiver also may be separate from the the transmitter at a fixed known position with respect to the source. In both instances, the distance may be deduced from the time measurement, the known speed of sound in the medium in which the burst has travelled, and the known relative positions of the source and receiver.
There are, however, significant problems which must be overcome before accurate distance measurements may be made using such acoustic ranging apparatus. One of the problems is that acoustic ranging apparatus are not only sensitive to acoustic energy which has been directed toward, or reflected from, targeted objects, but they are also sensitive to various forms of spurious signals which can give erroneous distance readings. These spurious signals may be due to stray reflections from other objects in the vicinity of the source or in the vicinity of the targeted object. One of the most significant sources of such spurious signals is reflection of acoustic energy by objects positioned to intercept energy in a series of side lobes situated off the axis of the main burst of acoustic energy, which are normally produced by the available acoustic energy transducers. Spurious signals may also be due to other acoustic energy sources in the measuring environment or to noise produced by the electronics in the ranging apparatus. These spurious signal sources produce false echoes in the ranging apparatus which are not accurate indications of the distance that the ranging apparatus is attempting to measure. Discrimination circuitry must, therefore, be included in the ranging apparatus to discriminate between true and false echoes. In its simplest form, this usually is a circuit producing a fixed threshold signal related to the noise level and a means for comparing the fixed threshold to the received signals. Those signals which are true echoes are above the threshold and are processed as such. Those signals which are below the threshold are ignored.
This type of an arrangement is subject to an additional complication due to the fact that acoustic energy becomes attenuated with distance travelled in the medium. This attenuation is caused by two factors. First, the acoustic energy is attenuated because it spreads in space as it travels away from the source. This attenuation is related to the inverse of the fourth power of the distance to a reflective object in a system where acoustic energy is reflected from that object and returned to the source. Second, the energy is attenuated even further because the medium through which the acoustic energy travels, usually air or water, is a lossy medium. This attenuation is generally an exponential function of distance travelled through the medium. The circuitry in the ranging apparatus must compensate for this attenuation if it is to be equally responsive to acoustic energy that has travelled different distances in the medium and if it is to be able to distinguish between false echoes from spurious signal sources and valid echoes from target objects. This has been accomplished in the past by changing the gain of the receiver as a function of time, the change of gain having had some relationship to the attenuation of sound in the medium. In this regard, the gain was increased as a function of time to compensate in some measure the decrease in amplitude of the acoustic energy as a function of distance travelled in the medium. See, for example, U.S. Pat. Nos. 4,000,650, 4,145,741, 4,197,528, 4,451,909, 4,464,738, 4,644,513, 4,675,854, 4,706,227, and 4,731,762.
These variable gain ranging systems are afflicted with significant disadvantages. They involve relatively high current and high power circuitry making them expensive and impractical to implement. They involve changing the gain of the ranging apparatus which cannot take place instantaneously thus making it difficult to capably distinguish between true and false echoes. The components used in a variable gain ranging apparatus are usually non-linear elements which make it even more difficult to obtain accurate distance readings. Also, the rejection of spurious echoes due, for example, to side lobes produced by the acoustic energy source, was not very good in variable gain systems. This is because it is difficult to economically produce the required gain as a function of time using analog circuitry, particularly because it is a complicated and costly task to increase the gain of an analog circuit as an inverse function of the attenuation of sound in air or other similar medium. Digital circuitry has been used in an attempt to reduce the cost of increasing the gain of the ranging apparatus, but the best that can be achieved is a stepped gain function, or some other piecewise linear approximation, which cannot accurately be a reflection of the inverse of the attenuation of sound in air. In addition, digital synthesis of the required gain function and changing the gain of the ranging apparatus in accordance with that digitally synthesized function can produce transients in the circuitry of the ranging apparatus thus making it difficult to obtain accurate distance readings.
One attempt to solve at least these some of these problems involved a constant gain ranging apparatus described in U.S. Pat. No. 4,315,325. A received signal is compared with a threshold voltage from a threshold amplifier. The threshold voltage is said to decrease as a function of time in accordance with the predicted attenuation of sound in the medium. The inputs of a comparator are connected to the received signal and the threshold voltage. A positive feedback loop is established between the output of the comparator and the threshold voltage generator to periodically switch the polarity of the threshold voltage in response to the results of the comparison. This arrangement is undesirable because the positive feedback will cause the circuit to oscillate which will make it difficult, if not impossible, to obtain any useful distance information. Also, the switching of the threshold voltage will cause unwanted noise to propagate throughout the entire circuit. Furthermore, important characteristics of the threshold voltage, such as the initial value, the final value, and the time constant, are not adequately specified. The patent only states that the threshold is to be a fraction of the expected return amplitude, which would make it impossible to reject a significant number of false echoes due to spurious signal sources.
Time varying thresholds with constant gains have been tried in radar receiving circuitry, but these are not applicable in an acoustic ranging apparatus because attenuation of the signal in a radar system differs quite a bit from the attenuation of sound in an acoustic ranging apparatus. Specifically, radar signals travel through an essentially lossless medium which simplifies the problems of compensating for the attenuation of those signals. See, for example, U.S. Pat. Nos. 4,023,168, 4,079,376, and 4,169,263.
There thus has been a long felt and unsatisfied need for an acoustic ranging apparatus which avoids all of these problems and which is simple and inexpensive to manufacture. Applicants have unexpectedly satisfied this need by not only solving all of the problems of the prior approaches, but also by having done so in a manner which is relatively simple, inexpensive, and economical. Specifically, Applicants have developed a constant gain thresholding arrangement in which a first order linear circuit is used to produce a threshold signal which varies in a manner which simulates the attenuation of sound as a function of distance travelled in the medium and which is used with other circuitry to produce a simple and inexpensive ranging apparatus which is accurate and is insensitive to spurious signals from a variety of sources.
It is an object of the invention to provide a distance measuring apparatus and method which accurately determines distance between two points and is capable of discriminating between true and false indications of distance.
It is an additional object of the invention to provide an acoustic ranging apparatus and method which avoids the problems of prior acoustic ranging apparatus and methods and does so at a reasonable cost.
Other objects and advantages are either specifically described elsewhere in this application or are apparent from that description.